The butadiene is an important chemical to be used as a raw material for synthetic rubbers and the synthetic rubbers are used in a large quantity for automobile tires and the like. Almost all of the butadiene is produced by a cracking process of oil. It is produced by extracting and separating butadiene from C4 fractions comprising isobutene, n-butene, butadiene, and the like. A demand for energy-saving type automobile tires using butadiene as a raw material has rapidly increased owing to global augmentation in demand for automobiles and increase in awareness of environmental issues in recent years. Further, the demand for butadiene has continued to greatly exceed the production from the C4 fraction owing to decrease in operation rate of naphtha crackers and use of petrochemical raw materials derived from natural gas, and therefore, a development of a new production process of butadiene has been desired.
There is known a process for producing butadiene from n-butene that remains from the extraction of butadiene. For example, in Patent Documents 1 and 2, processes of oxidative dehydrogenation in the presence of a complex metal oxide catalyst containing molybdenum, bismuth, iron, and cobalt as main ingredients are described. However, from the standpoints of catalyst activity, selectivity for butadiene, stability of reaction operation, catalyst life, catalyst production, and the like, conventional catalysts are industrially insufficient, and therefore, an improvement thereof has been desired.
The catalyst life of the reaction and the instability of the reaction operation are considered to be due to the carbon content that accumulates on the catalyst. In Patent Document 3, it is described that the accumulation of the carbon content can be suppressed by diluting the catalyst with an inert solid matter and thereby controlling a successive reaction of the product. However, the accumulation of the carbon content cannot be sufficiently suppressed by this method, and besides, a reduction of yield accompanying a decrease in an amount of an active ingredient occurs and a cost for mixing the active ingredient with the inert solid matter takes, and therefore, this method is industrially disadvantageous.
In Patent Document 4, it is described that a high conversion and a high selectivity are maintained for a long period of time by mixing a slight amount of silica into a known catalyst-active ingredient composition, but it has a disadvantage that the catalyst is difficult to produce stably in an industrial scale.
Moreover, in Patent Document 5, there is a description with respect to a coat-shaped catalyst, in which a pore-forming agent is mixed into a catalyst precursor, as well as the production thereof in an industrial scale. However, there is no clear description with respect to an effect on the conversion of butene and the selectivity for butadiene resulting from the production of the coat-shaped catalyst at which a pore-forming agent is mixed.
Thus, from the standpoints of catalyst performance, catalyst life, and operation control in a plant, conventional catalysts do not necessarily afford sufficient performance as industrial catalysts, and further improvement thereof has been desired.